1. Field of the Invention
The present invention relates to a signal processing method and a displacement information measuring device utilizing the same, adapted, for example, for use in a rotary encoder or a linear encoder or the like for detecting the displacement information of a movable object, having an optical scale provided with periodically arranged plural gratings with V-shaped grooves on an external or internal periphery of a cylindrical article or on a flat article, by introducing a light beam into said optical scale and utilizing the light beam obtained therefrom.
2. Related Background Art
Photoelectric rotary or linear encoder have been utilized for detecting the revolution of a rotating mechanism, the variation in said revolution or the displacement of a linearly moving object, for example in computer-related equipment such as floppy disk drives, office equipment such as printers, NC machine tools, or capstan motor or rotary drum of the video cassette recorders.
FIG. 1 is a partial schematic view of a rotary encoder utilizing so-called Talbot's interference, as disclosed in the Japanese Patent Laid-open Application Nos. 61-10716 and 1-176914.
A semiconductor laser 1 emits a coherent light beam of a wavelength .lambda.. A collimating lens system 2 converts the diverging light beam from the semiconductor laser 1 into parallel light beam, and said semiconductor laser 1 and said colliminating lens system 2 constitute an irradiating means LR. A cylindrical optical scale 3, having a translucent grating with periodic plural V-shaped grooves on the internal periphery, is rotated as indicated by an arrow.
Said optical scale 3 is composed of a translucent optical material. Opposed to the irradiating means LR across the optical scale 3, there are provided three photodetectors 4a, 4b, 4c constituting light-receiving means 4. The outputs of said photodetectors are connected to a signal processing circuit 5, which is provided with a pulse counting circuit, a rotating direction judging circuit, a signal interpolation circuit and the like.
In the illustrated rotary encoder, the light beam from the irradiating means LR is incident on an area 3a of the optical scale 3, and the light beam subjected to optical modulation (diffraction) by said optical scale 3 is further introduced another area 3b of said optical scale 3 to employ optical modulation (deflection). The plural (three) light beams emerging from said optical scale 3 are received by the light-receiving means 4, of which output signals are utilized for detecting the rotation information of the optical scale 3.
In the conventional rotary encoder, the signal processing circuit 5 obtains desired binary signals CA, CC from the output signals SA, SC from the light-receiving means 4, by selecting an arbitrary comparison level by an external reference voltage V1 as shown in FIGS. 2A and 2B.
However the output signals of the light-receiving means 4 may involve fluctuations in the DC level and in the signal amplitude, because of the temperature characteristics of the sensitivity of the photodetectors, variation in the light amount resulting from time-dependent decrease in the light emission intensity of the laser, fluctuation in the light emission intensity of the laser resulting from a variation in the ambient temperature, variation in the optical transmittance of the optical scale 3 resulting from dusts or smear thereon etc.
As a result, there appears a variation of the duty ratio, as shown in FIGS. 3A and 3B, in case of obtaining the binary signals CA, CC from the output signals SA, SC with an arbitrary constant comparator level determined by-the external reference voltage V1.